Method of treating zinciferous surfaces



States METHOD OF TREATING ZINCIFEROUS SURFACES No Drawin Application April 28, 1954, Serial No. 426,300

8 Claims. (Cl. 148-62) This invention relates to a process and solutions for the substantially colorless passivation of surfaces of articles of, or plated with, zinc and its alloys. Processes of the present invention are particularly useful in the treatment of zinc surfaces which are customarily called galvanized.

It is well known that zinciferous surfaces and particularly galvanized surfaces become corroded in a relatively short time even in ordinary atmospheres and where the atmosphere contains salt or corrosive contaminants, as is true in seashore and industrial areas, the rate of corrosion becomes quite rapid and severe. In fact, where articles are galvanized with modern day zinc-aluminum alloys the surfaces are so susceptible to corrosion that, quite often, a severe corrosion problem is encountered in the galvanizing mill itself.

In order to avoid or minimize the corrosion of zinciferous surfaces it has been common practice heretofore to provide the surface with a protective film. However, most of the known surface treatment solutions useful for this purpose consist of chromic acid and various other ingredients, such as chlorides and sulphates, and these are generally objectionable because they impart a dark color to the zinciferous surface, which color is often objectionable, particularly on a galvanized surface. In order to eliminate this objectionable color, cumbersome chemical treatments have been developed in an effort to leach the color from the protective film. In addition to such efforts and, more recently, passivation has been attempted by means of the use of a dilute solution of chromic acid and silicates but even where such processes are successful in producing a relatively colorless passivated surface on the zinc, the time involved in securing results has precluded the acceptance of such processes in modern, high speed manufacturing operations such as in the galvanizing mill itself in which connection it must be borne in mind that, in modern continuously operated galvanizing mills, the maxi-mum span of time which may be allotted to a passivating treatment is a matter of seconds.

With the foregoing in mind, the principal object of the present invention resides in the provision of an improved method and materials for the treatment of zinciferous surfaces which will yield a very high degree of passivity without substantially altering the original physical appearance of the surface under treatment and in connection with this object it is our purpose to provide such a method and materials which are particularly useful in the passivation of galvanized surfaces even where such surfaces contain substantial quantities of aluminum, i. e., as much as 4% or more. a

A further object of the invention is the provision of an improved process and materials which will produce a passivated substantially colorless surface on zinc in a period of from 2 to 10 seconds.

A still further object of the invention is the provision of a. process and materials which are capable of producing a passivated zinciferous surface which is in excellent condition to receive a final siccative finish of paint, lacaten-t Cil ice

quer, japan or the like. Other objects of the invention will become apparent in connection with the following detailed description.

The present invention is based upon the discovery that if a zinciferous metal surface is subjected to the action of an aqueous acidulated solution containing hexavalent chromium, chloride and complex fluoride that it is possible to passivate such a surface without substantially altering its physical appearance in a treating time of between 2 and 10 seconds.

The amount of hexavalent chromium in the bath is important. There should be a minimum of 20 grams per liter of hexavalent chromium, calculated as chromic acid (CrOs) and as a practical maximum 90 grams per liter. Preferably the solution should contain from 40 to grams per liter. At concentrations below 20 grams per liter, calculated as CIOs, passivity of the treated metallic surface is greatly diminished. The maximum amount of hexavalent chromium present is dictated by economy and the solubility of the source of the hexavalent chromium. However, as a practical maximum we prefer not to use more than grams per liter. We have found that approximately 40 to 75 grams per liter of hexavalent chromium in the bath will produce excellent results. The form in which the hexavalent chromium is introduced into the bath makes little or no difference. For instance, it may be introduced as chromic acid, soluble salt-s of chromic acid such as potassium chromate, sodium dichromate, ammonium chromate, dichromate, etc.

The amount of chloride in the bath is also important. For each gram of hexavalent chromium present in the bath (calculated as CrOs) there should be at least 0.25 gram of chloride calculated as sodium chloride. Preferably the amount of chloride in the bath should be approximately equal to the chromate concentration and we prefer not to use more than double the amount of chloride in comparison with hexavalent chromium in the bath. The form in which the chloride is introduced makes little or no difference, as long as it is introduced in such a fashion as to cause no reduction of hexavalent chromium to trivalent chromium. For instance, it may be introduced as hydrochloric acid, sodium chloride, ammonium chloride, etc.

The minimum amount of complex fluoride in the bath is also important. There should be not less than 0.4 gram per liter complex fluoride in the bath, calculated as fluorine. We prefer to use from approximately 0.9 to 1.8 grams per liter fluoride in the bath, calculated as fluorine. The maximum amount of fluoride in the bath is not critical and any amount which is capable of going into solution is satisfactory. Beyond this any excess will act as a reservoir. As a source of complex fluoride we have found that fluosilicic acid, fluoboric acid, fluozirconic acid, fluostannic acid, fluotitanic acid and their soluble salts are all satisfactory. For some reason not fully understood we believe that the fluotitanates lead to optimum results.

The temperature of the bath is also important. The minimum operating temperature should be in the neighborhood of F. and the maximum is the boiling point. We prefer to operate in the neighborhood of F. At temperatures below 160 F. the passivity imparted to the treated surface falls markedly. At temperatures above F. in some instances we have found that on some galvanized surfaces the treated solutions may tend to produce a small amount of loose powder on the metallic surface and that the chemical action on the surface may be such as to lead to a matte dulling effect of the surface. At temperatures of around 190 F. optimum passivity with no color change of the treated surface is usually achieved.

The pH of the operating solution is also critical. The

pH must be between 1.9 and 2.8 and preferably between 2.0 and 2.4. At pHs lower than 1.9 there is a marked tendency of the bath to produce undesirable color of the treated work. At pHs greater than 2.8 little or no passivity is achieved. Optimum passivity with minimal color change is achieved with solutions having a pH of between.2.0 and 2.4.

The time of treatment is important and must be between 2 and seconds and preferably between 4 and 7 seconds. At treating times of less than 2 seconds little passivity is achieved and with treating times of over 10 seconds there is a marked tendency for undesirable color to develop. In general, we have found that the optimum treating time is in the neighborhood of 4 to 7 seconds.

In general, 'it may be said that for the longer treating times, i. e., in the neighborhod of 10 seconds it is generally preferable to work with solutions having a relatively high pH and a fairly low bath operating tem perature. At short treatment times, i. e., in the order of 2 seconds, the pH of the solution should be low and the temperature high. Also, it is to be noted that for the more lengthy treating cycles more dilute solutions are preferred and the converse is true for the short treating times we prefer to use the more concentrated solutions. In general, for short treating times high temperatures are preferred, whereas for longer treating times lower temperatures are preferred. When working on the fringes of one or more of the limits previously specified on some types of galvanized surfaces, either undesirable color or lack of passivation may occur. In such instances it is merely necessary to readjust one or more of the process variables to more closely approximate the optimum values previously given, as we have found that within these limits excellent passivation with no visual change in the surface characteristics of the treated metal is achieved.

Examples of suitable solutions for use in our process are as follows:

Example] Grams K2C1207 70 NaCl 70 KzTiFeHzO 2 The pH of the solution is adjusted to 2.5 with hydrochloric acid.

' Example 2 Grams CI'OS NaCl 45 'KzTiFeHzo 2 The pH of the solution is adjusted to 2.5 with sodium hydroxide.

Example 3 Grams 'NazCm'OmZHzO 70 NH4C1 45 NaaSiFs 2 The pH of the solution is adjusted to 2.5 with hydrochloric acid.

Example4 e Grams ClOs 90 NaCl 90 K2TiFe.H2O 4 The pH of the solution is adjusted to 2.5 with sodium hydroxide.

ExampleS Grams CrOs 45 NH4CI 40 NaBF4 2 The pH of the solution is adjusted to 2.5 with sodium hydroxide.

The pH of such a solution without adjustment is usually in the neighborhood of 4.5 and should be adjusted with l-lCl to approximately 1.8 or lower.

In using solutions of the type illustrated for the process of the present invention it is necessary merely that they be heated to operating temperature, i. e., between F. and 195 F. and preferably to approximately 195 F. The solutions are then allowed to contact the metal surface for a period of from 2 to 10 seconds as previously indicated. The surface should be reasonably clean but since the cleaning forms no part of the present invention it will not be set forth in detail because any cleaning procedure for zinciferous surfaces which is familiar to the art. can be adopted with the present invention. In situations where the present invention is used in conjunction with a continuous galvanizing line, after the galvanizing procedure, the metal is usually simply run through either a spray setup or a dip tank or subjected to the action of the solution by any-other suitable means of applying the solution to the surface just so long as the metal is exposed to the action of the solution for the appropriate length of time as above described. Following the. treatment as just described, obviously the adhering treating solution should be removed by means of a Water rinse.

We claim:

1. The method of passivating a zinciferous surface which comprises treating it with an aqueous acidulated solution consisting essentially of hexavalent chromium, chloride and complex fluoride and water, the said ingredients per liter of solution being present according to the following formula:

chromic acid said solution having a pH lying between 1.9 and 2.8 and a temperature lying between 160 F. and 195 F. and the length of time of the treatment being not less than 2 nor more than 10 seconds.

2. The method of claim 1 wherein the hexavalent chromium content lies between 40 and 75 grams per liter, the chloride content is approximately equal to the hexavalent chromium content, and the complex fluoride content lies between 0.9 and 1.8 grams per liter.

3. The method of claim 1 wherein the chloride content is approximately equal to the hexavalent chromium content.

4. The method of claim 1 wherein the hexavalent chromium content lies between 40 and 75 grams per liter.

5. The method of claim 1 wherein the complex fluoride content lies between 0.9 and 1.8 grams per liter.

6. The method of claim 1 wherein the complex fluoride in the solution is supplied by fluotitanates and further wherein the temperature is maintained at substantially F.

7. A material for the colorless passivation of zinciferous surfaces consisting essentially of a solution constitutegl as described in claim 2 but heated to substantially 190 F.

8. The method of claim 1. wherein the complex fiuo ride is chosen from the group consisting of fluosilicic,

fluoboric, fiuozirconic, fluostannic and fluotitanic acids 2,393,943 Thomas et a1. Jan. 29, 1946 and the soluble salts of such acids. 2,494,909 Spruance et a1. Jan. 17, 1950 2,524,577 Stareck Oct. 3, 1950 References Cited in the file Of this patent 2,762,731 Heller et a1. Sept. 11, 1956 UNITED STATES PATENTS 5 FOREIGN PATENTS 2,106,904 Wilhelm 1938 675,334 Germany May 6, 1939 2,114,151 Romig Apr. 12, 1938 2,276,353 Thompson Mar. 17, 1942 

1. THE METHOD OF PASSIVATING A ZINCIFEROUS SURFACE WHICH COMPRISES TREATING IT WITH AN AWUEOUS ACIDULATED SOLUTION CONSISTING ESSENTIALLY OF HEXAVALENT CHROMIUM, CHLORIDE AND COMPLEX FLUORIDE AND WATER, THE SAID INGREDIENTS PER LITER OF SOLUTION BEING PRESENT ACCORDING TO THE FOLLOWING FORMULA: 